The present invention relates to a method and apparatus for examining tissue in order to differentiate the examined tissue from other tissue according to the dielectric properties of the examined tissue. The invention is particularly useful to differentiate cancerous tissue, particularly breast cancer tissue, from normal, healthy tissue, and is therefore described below particularly with respect to this application.
Breast cancer is the second leading cause of cancer deaths in women today (after lung cancer) and is the second most common form of cancer among women (after skin cancer). According to the World Health Organization, more than 1.2 million people will be diagnosed with breast cancer this year worldwide. The American Cancer Society estimates that in 2001, approximately 192,200 new cases of invasive breast cancer (Stages I-IV) will be diagnosed among women in the United States; and another 46,400 women will be diagnosed with ductal carcinoma in situ (DCIS), a non-invasive breast cancer. Though much less common, breast cancer also occurs in men, it being estimated that 1,500 cases will be diagnosed in men in 2001. It is further estimated that 40,600 deaths will occur in 2001 from breast cancer (40,200 among women, 400 among men) in the United States. The incidence rate of breast cancer (number of new breast cancers per 100,000 women) increased by approximately 4% during the 1980s but leveled off, to 100.6 cases per 100,000 women, in the 1990s. The death rates from breast cancer also declined significantly between 1992 and 1996, with the largest decreases being among younger women. Medical experts attribute the decline in breast cancer deaths to earlier detection and more effective treatments.
Mammography is currently the best available screening modality for early detection of breast cancer. If the mammography finds a subspecies legion, the individual is directed to undergo a biopsy or other advanced screening methods, like ultrasound or MRI CT etc. Only 20% of the women that undergo a biopsy proceed to a surgical treatment. The traditional method for histological confirmation involves open surgery biopsy. An alternative is image guided biopsy, which is less invasive and more costly. The total number of breast biopsies in the U.S. is about 1.2 M per year. The open biopsy itself is a surgical procedure in which the breast is open and the tumor or lump is taken out, preferably fully.
The traditional method of biopsy, however, is not always successful and fails to successfully remove the appropriate lesion in about 0.5-17% of the cases. Some of the reasons given for unsuccessful biopsies include: 1) poor radiological placement of the localization wire; 2) preoperative and intraoperative dislodgment of the wire; 3) surgical inaccuracy and inadequacy in excising the appropriate tissue; 4) failure to obtain a specimen radiograph; and 5) failure by the pathologist to locate the focus of the disease when searching through a larger tissue sample provided by the surgeon.
All of the above reasons stem from a fundamental problem that during the surgery, the surgeon does not have a real time indication or delineation of the tumor. Because of the difficulty in precisely delineating the cancerous tissue, the surgeon may cut out more than was really necessary to better assure that the entire tumor was removed.
Today, women with stage I and stage II breast cancer are candidates for treatment with modified radical mastectomy and with immediate reconstruction. Breast-conserving therapy (BCT) is also available. Breast conservation therapy consists of surgical removal of a breast nodule and of the auxiliary fat pad containing the auxiliary lymph nodes (about a quarter of the breast). This is followed by radiation therapy to the breast and auxiliary areas in some cases. In this type of operation, precise margin assessment or delineation of the cancerous tissue during the operation is crucial to the success of the procedure since the goal is to remove the tumor completely while minimizing damage to the breast.
This trade-off between complete removal of the tumor, and conservation of the breast, is usually difficult to optimize because the surgeon generally does not know the actual margins of the tumor. If the surgeon were able to clearly delineate the tumor margins during the operation by an on-line margin detector, this trade-off could be better optimized.
The ability of recognizing cancer cells, and especially breast cancer cells, using bioimpedance is well established in the biomedical literature5,6,7,8. The usual method for measuring bioimpedance is by introducing a sample into a special chamber and applying an AC current through it while recording the voltage across the sample at each frequency9,10. More modern methods rely on multiple electrode matrices which are connected with the human body and measure physiological and pathological changes. Some of the methods aim to localize tumor cells inside the human body and to form an image11,12. Although this method is approved by the FDA, it lacks the necessary accuracy for a screening device mainly because of the inherent limitations of long wavelengths and noise from the contact electrodes.
Another technique, based on magnetic13 bioimpedance, measures the bioimpedance by magnetic induction. This technique consists of a single coil acting as both an electromagnetic source and a receiver operating typically in the frequency range 1-10 MHz. When the coil is placed in a fixed-geometric relationship to a conducting body, the alternating electric field in the coil generates electrical eddy current. A change in the bioimpedance induces changes in the eddy current, and as a result, a change in the magnetic field of those eddy currents. The coil acts as a receiver to detect such changes. Experiments with this technique achieved sensitivity of 95%, and specificity of 69%, distinguishing between 1% metastasis tumor and 20% metastasis tumor. Distinguishing between tumor and normal tissue is even better.
Although the exact mechanism responsible for tissue impedance at certain frequencies is not completely understood, the general mechanism14,15 is well explained by semi-empirical models that are supported by experiments16,17,18.
Variations in electrical impedance of the human tissue are described in the patent literature to provide indications of tumors, lesions and other abnormalities. For example, U.S. Pat. Nos. 4,291,708; 4,458,694; 4,537,203; 4,617,939 and 4,539,640 exemplify prior art systems for tissue characterization by using multi-element probes which are pressed against the skin of the patient and measure impedance of the tissue to generate a two-dimensional impedance map. Other prior techniques of this type are described in WO 01/43630; U.S. Pat. No. 4,291,708 and U.S. Pat. No. 5,143,079. However, the above devices use a set of electrodes that must be electrically contacted with the tissue or body, and therefore the contact is usually a source of noise and also limits maneuverability of the probe over the organ.
Other prior patents, for example U.S. Pat. Nos. 5,807,257; 5,704,355 and 6,061,589 use millimeter and microwave devices to measure bioimpedance and to detect abnormal tissue. These methods direct a free propagating radiation, or a guided radiation via waveguide, onto the organ. The radiation is focused on a relatively small volume inside the organ, and the reflected radiation is then measured. However, these methods lack accuracy and spatial resolution since they are limited by the diffraction limit.
Another prior art technique is based on measurement of the resonance frequency of a resonator as influenced by the tissue impedance. This technique also uses radiation from an antenna, usually a small dipole antenna attached to a coaxial line. Although non-contact, the device actually measures average values inside the organ, and its ability to detect small tumor is doubtful. Similar prior art is described in Xu, Y., et al. xe2x80x9cTheoretical and Experimental Study of Measurement of Microwave Permitivity using Open Ended Elliptical Coaxial Probesxe2x80x9d. IEEE Trans AP-40(1), January 1992, pp 143-150.3. U.S. Pat. No. 6,109,270 (2000 NASA) describes a measurement concept with a multi-modality instrument for tissue identification in real-time neuro-surgical applications.
Other known prior art includes an open-ended coaxial2,3,4 probe having a center conducting wire surrounding by an insulator and enclosed in an external shield. This type of tip generates both a near field evanescent wave and a far field propagating wave. The propagating wave is undesirable because it interferes with the near field evanescent wave. In order to minimizes the propagating wave, researchers attempted to use coaxial cable with smaller and smaller diameters. But eventually large energy losses and difficult construction limited this direction.
Other existing medical instruments provide general diagnoses for the detection of interfaces between different types of tissues, such as cancerous tissue and healthy tissue, etc. However, such detections have been limited clinically to pre-operative scans, or demand large scanning multi-million-dollar machines, like the MRI CT Mammography. Furthermore, real-time attempts to use these detecting methods are very sensitive to movement of the body, and cannot really be used to position the cutting knife or the biopsy needle. Existing devices provide diagnostic data of limited use since the tissue sampled or removed depends entirely upon the accuracy with which the localization provided by the preoperative CT or MRI Us scan is translated to the intracranial biopsy site. Any movement of the organ or the localization device results in an error in biopsy localization. Also, no information about the tissue being cut by the needle or knife is provided.
Detecting breast cancer tissues by measuring biompedance is thus well established, and the ability of this technique for delineating cancerous cells inside the body has been proved. However, there is currently no reliable real-time bioimpedance measuring device of sufficiently high accuracy for local tissue characterization and of a spatial resolution comparable to that provided by mammography.
An object of the present invention is to provide a method, and also a system, having advantages in one or more of the above respects for examining tissue in order to differentiate the examined tissue from other tissue according to the dielectric properties of the examined tissue. Another object of the invention is to provide a method and system enabling more precise differentiation in a real-time manner of cancerous tissue from healthy, normal tissue.
According to one broad aspect of the present invention, there is provided a method of examining tissue in order to differentiate it from other tissue according to the dielectric properties of the examined tissue, comprising: applying an electrical pulse to the tissue to be examined via a probe such that the probe generates an electrical fringe field in the examined tissue and produces a reflected pulse therefrom, with negligible radiation penetrating into other tissues or biological bodies near the examined tissue; detecting the reflected electrical pulse; and comparing electrical characteristics of the reflected electrical pulse with respect to the applied electrical pulse to provide an indication of the dielectric properties of the examined tissue.
According to one broad aspect of the present invention, there is provided a method of examining tissue in order to differentiate it from other tissue according to the dielectric properties of the examined tissue, comprising: applying an electrical pulse to the tissue to be examined via a probe formed with an open cavity such that the probe generates an electrical fringe field in the examined tissue within said cavity and produces a reflected pulse therefrom, with negligible radiation penetrating into other tissues or biological bodies near the examined tissue; detecting the reflected electrical pulse; and comparing electrical characteristics of the reflected electrical pulse with respect to the applied electrical pulse to provide an indication of the dielectric properties of the examined tissue.
The electrical fringe field is an electrical field that exists at the edges of a charged conductor. Usually an electrical fringe field is a DC field, but in the present case, it is a time-dependent field since its source is a voltage pulse. The open cavity defined by the inner and outer conductors serves as a small capacitor probe in which the electrical fringing field is generated between the inner and outer conductors. When a pulse is transmitted through the transmission line to the probe open cavity of the probe closed by the tissue being examined, the pulse is reflected back to the transmission line. Generally speaking, the reflection depends on the impedance of the region at the open cavity of the probe, which impedance depends on the dielectric properties of the examined tissue closing the open end of the cavity. Accordingly, the reflected pulse carries with it information about the dielectric properties of the examined tissue. These properties produce a change in the time-domain-profile of the reflected pulse.
The electrical characteristics of the reflected electrical pulse are compared with those of the applied (incident) electrical pulse by sampling both electrical pulses at a plurality of spaced time intervals, e.g., every 0.2 nanoseconds, and comparing the voltage magnitudes of the two electrical pulses at the spaced time intervals. Both pulses are then transformed by a FFT function to the frequency domain, i.e., amplitude and phase for each frequency. The reflection coefficient is then calculated in the frequency domain; and the frequency dependent complex impedance of the tissue is then calculated using the theoretical relation between impedance and reflection.
It will thus be seen that when the examined tissue is placed in the region of the open cavity define by the inner and outer conductors of the probe, the electrical fringe field penetrates into the open cavity. This penetration is due to the relatively low conductivity of the tissue. Since the electrical fringe field penetrates the tissue, the build-up profile of the electrical fringe field depends on the dielectrical properties of the tissue which produce the changes in the reflected electrical pulse generated by the application of the applied (incident) electrical pulse to the open cavity.
According to further features in the preferred embodiments of the invention described below, the inner conductor includes a tip within the open cavity, which tip is formed with at least two different diameters for enhancing the electrical fringe field. Preferably, the tip of the inner conductor carries a plurality of sharp, thin, electrically-conductive projections, or spikes, for enhancing the electrical fringe field. In the preferred embodiment described below the thickness of these projections or spikes, when used, is from a few microns to about 200 microns.
Preferably, the above-obtained electrical characteristics of the examined tissue are compared with previously stored dielectric projections of known normal and cancerous tissues to constitute a first level of characterization of the examined tissue. A second level of characterization of the examined tissue may be effected to reduce ambiguities by comparing the Cole-Cole parameters of the examined tissue with those previously stored of known normal and cancerous tissues. A third level of characterization of the examined tissue may be effected to further reduce ambiguities by comparing similarities between three-dimensional curves of the examined tissue with those previously stored of known normal and cancerous tissues.
As will be described more particularly below, the method of the present invention, being based on the generation of an electrical fringe field with negligible radiation penetrating into the tissue itself, eliminates almost completely the propagating wave, while the evanescent wave reflections are reduced significantly.
The method of the present invention is thus to be distinguished from prior art, such as U.S. Pat. No. 6,173,604, which utilizes a scanning microwave microscope having a sharpened conducting tip extending through the end wall of a resonator, to reduce the effect of the propagating wave. Such a known probe, cannot be used to measure biological tissue without harming the tissue itself; furthermore, the evanescent wave of such a known probe will penetrate into the whole human body. Also, since such a known technique relies on average power measurement, and not on voltage measurement, it is not able to calculate dielectric properties in the time domain. Also, the frequency range in such method is in the microwave region of the electromagnetic spectrum. Further differences are that it is not flexible and cannot be hand-held.
Still further features and advantages of the invention will be apparent from the description below.